Conventional apparatus and methods for the measurement of thermal diffusivity of solids generally require application of heat at a point in the solid using some type of heater, and then measurement of temperature rises as a function of time at several points within the solid using an appropriate temperature measuring device such as a thermistor. Recent developments in thermooptical techniques include use of a pulsed laser beam to supply heat to the sample material. However, temperature measurements are still generally made using thermistors or other conventional devices. In one method, a single laser beam is used to both form the thermal lens and to monitor the change of refractive index in a sample material. This single beam technique utilized a helium/neon laser whose output was periodically interrupted with a mechanical shutter. Use of a single beam, although superior to many conventional methods, has limited resolution capability because the measurement beam is also the beam supplying heat to the sample material.
Problems associated with conventional methods for measuring thermal properties of a material are many and varied. Typically, a relatively large sample is required, and it is difficult to perform a measurement under quasi-isothermal conditions since temperature gradients of less than 0.01.degree. Kelvin per centimeter cannot be measured with precision. It is also difficult to adapt conventional techniques to samples under stress or to samples undergoing various other types of test environments. Conventional techniques tend to be slow, and transient changes in thermal properties cannot be measured utilizing conventional measurement apparatus. In addition, conventional techniques do not provide any way to directly measure rates of lattice relaxation processes in polymers and composites or to accurately measure anisotropic thermal properties or thermal properties of a surface of a material. An apparatus and method according to the present invention solves all of the above problems associated with conventional measurement techniques.